1. Field of the Invention
The present invention relates to a fixing apparatus for fixing a toner image onto a recording material, and an image forming apparatus having the same.
2. Description of the Related Art
As a fixing apparatus for use in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing apparatus of heat-roller fixing type has been in wide use. FIG. 7 is a front view showing a configuration example of a conventional heat-roller fixing-type fixing apparatus 70 in simplified form. The heat-roller fixing-type fixing apparatus 70 includes a pair of rollers, to be more specific, a fixing roller 71 and a pressure roller 72, that are brought into contact with each other under pressure. By means of a heating section composed for example of a halogen heater, which is placed in each of or one of the pair of rollers interiorly thereof, for example, a heater lamp 73 placed inside the fixing roller 71, the pair of rollers are heated to a predetermined fixing temperature. With the pair of rollers kept in a heated state, a recording sheet P, which is a recording material having formed thereon an unfixed toner image T, is fed to a region where the pair of rollers make pressure-contact with each other, namely a so-called fixing nip region N1. Upon the recording sheet P passing through the fixing nip region N1, the toner image is fixed into place under application of heat and pressure.
Moreover, a fixing apparatus for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller 71. By designing the fixing roller 71 as an elastic roller, it is possible for the surface of the fixing roller 71 to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller 71 makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixation on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region N1, it is possible to provide enhanced mold releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region N1 is convexly curved in a radially-outward direction so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability. That is, a paper stripping action can be produced without using a stripping portion such as a stripping paw1 (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
In the fixing apparatus 70 thus constructed, an increase of a fixing nip width, namely the width of the fixing nip region N1, may be considered as an attempt to keep up with higher and higher process speeds. The examples of methods for increasing the fixing nip width include an increase in the thickness of the elastic layer of the fixing roller 71 and an increase in the diameter of the fixing roller 71.
In the fixing roller 71 having the elastic layer, however, the elastic layer in itself exhibits extremely low thermal conductivity. Therefore, an increase in the thickness of the elastic layer may lead, in a case of providing a heating section within the fixing roller 71, to further reduction in heat transfer efficiency. In this case, if the process speed is increased, there is a possibility that the temperature of the fixing roller 71 cannot stay close to a fixing temperature. On the other hand, an increase in the diameter of the fixing roller 71 may lead to prolonged warm-up period of time, which results in higher power consumption.
In order to solve such problems, there has been proposed a technique to apply heat to the surface of the fixing roller 71 externally by means of a heating roller 75 which is brought into pressure-contact with the fixing roller 71 and has a heater lamp 74 disposed in it. In the fixing apparatus 70 thus constructed, on the basis of temperature data obtained through detection by, for example, a temperature detecting section 76 disposed in the vicinity of the outer peripheral surface of the heating roller 75 and a temperature detecting section 77 disposed in the vicinity of the outer peripheral surface of the fixing roller 71, temperature control is exercised over the heater lamps 73 and 74.
By way of the technique as described hereinabove, for example, Japanese Examined Patent Publication JP-B2 54-15215 (1979) discloses a toner-image fixing apparatus in which, as a thermally-fusing roller which is brought into contact with an image-bearing surface of a material for holding a toner image, a heat-insulating elastic roller is applied for use. Moreover, a heating roller having a heat source disposed thereinside is disposed so as to make pressure-contact with the elastic roller. The elastic roller is heated externally by this heating roller. The image holding material on which is borne a toner image is caused to pass through a region between the elastic roller and a rigid pressure-contact roller. In the toner-image fixing apparatus, in order to keep the surface temperature of the elastic roller substantially constant, the elastic roller is made smaller in diameter than the heating roller, and the preset temperature of the heating roller is changed to rise only at the time of the passage of the image holding material.
Moreover, Japanese Unexamined Patent Publication JP-A 50-62448 (1975) discloses a toner-image fixing apparatus in which, as a thermally-fusing roller which is brought into contact with a toner image-bearing surface of a material for holding a toner image, a heat-insulating elastic roller is applied for use. In a region between the elastic roller and a rigid pressure-contact roller whose diameter is larger than that of the elastic roller, the toner image holding material on which is borne a toner image is caused to pass. In the toner-image fixing apparatus, the surface of the elastic roller is heated externally by a heating roller whose diameter is larger than that of the elastic roller.
Further, Japanese Unexamined Patent Publication JP-A 52-64935 (1977) discloses a fixing apparatus in which a fixing roller having a silicone rubber layer is heated externally on contact with pressure-contact rollers having a heating source disposed thereinside disposed so as to have sandwiched therebetween the fixing roller in a vertical direction. In the fixing apparatus, the fixing roller is rotated dependently with the rotation of the pressure-contact roller without causing slipping.
Still further, Japanese Unexamined Patent Publication JP-A 54-33040 (1979) discloses a heat fixing apparatus in which a fixing roller having an elastic body is heated externally on pressure-contact with a heating roller and a pressure roller with a heating source disposed thereinside that are made larger in diameter than the fixing roller. In the heat fixing apparatus, the surface temperature of the pressure roller is detected and, on the basis of the result of detection, turn-on control is exercised to actuate the heater of the pressure roller and the heater of the heating roller at the same time.
In addition, Japanese Examined Patent Publication JP-B2 7-56583 (1995) discloses a heat fixing apparatus in which a fixing roller having a silicone rubber is heated externally on pressure-contact with an upper pressure roller and a lower pressure roller having a heating source disposed thereinside. In the heat fixing apparatus, selection between a full-color image copying mode and a monochromatic image copying mode is effected by switch operation. In accordance with the condition of unfixed toner in the selected mode, the heating temperatures of the upper and lower pressure rollers are controlled properly.
In the fixing apparatuses disclosed in JP-B2 54-15215, JP-A 50-62448, JP-A 52-64935, JP-A 54-33040, and JP-B2 7-56583, the fixing roller is heated externally by using the heating roller and the pressure roller. In such a construction, however, even if the heating roller for use is made larger in diameter than the fixing roller, as practiced in JP-B2 54-15215 and JP-A 50-62448, the dimension of a heating nip width, which is the width of the region where the fixing roller and the heating roller are kept in contact with each other, is inadequate. In consequence, the efficiency in heating of the fixing roller is so low that it becomes impossible to achieve a large increase in process speed.
Furthermore, in the absence of a heating section within the fixing roller, the fixing roller cannot be pre-heated evenly during standby. This gives rise to a problem of prolonged first copy output time (hereafter expressed as “FCOT”, which is a period of time taken to complete image formation for the first time around).